1. Field of the Invention
The present invention relates to a demodulation method and apparatus for a multicarrier transmission scheme in which a plurality of subcarriers are modulated individually and the thus-obtained plurality of modulated signals are transmitted over a single symbol duration T. The present invention is particularly effective for a demodulation apparatus which receives a signal modulated in accordance with, for example, orthogonal frequency division multiplexing (OFDM).
The present invention relates further to a multicarrier demodulation method and apparatus. The present invention is particularly effective for reception of OFDM signals at locations where delayed waves exert considerable influence.
2. Description of the Related Art
For example, in an OFDM scheme in which the frequency interval of N carriers is set to Δf (=1/T), where T is the length of a single symbol excepting a guard interval, on the modulation side, N complex digital modulated signals obtained through sampling at sampling intervals of 1/(NΔf) are generated by means of inverse discrete Fourier transform (IDFT). On the demodulation side, OFDM demodulation is performed through discrete Fourier transform (DFT) by use of N complex digital signals, which are obtained by sampling, at sampling intervals of 1/(NΔf), a signal from which the guard interval has been removed and which has the single symbol length T. Recently, a technique for reducing the number of sampling points of the received signal used in the above calculation; i.e., halving the single symbol length, has been reported (23rd Symposium on Information Theory and Its Applications, Oct. 2000, pp. 101-104).
Incidentally, on the modulation side, many nullcarriers, which are always zero, are generally contained in inputs (signals to be superposed on subcarriers) of the N-point IDFT (N-point inverse discrete Fourier transform).
For example, in the OFDM modulation scheme, a waveform called a guard interval is added before each effective symbol in order to prevent deterioration of orthogonality that would otherwise occur as a result of superposition of a delayed wave. For example, the last quarter of the effective symbol is added before the effective symbol so as to make a single symbol 5/4 times its original duration. This guard interval prevents the symbol from being influenced by the delayed wave during demodulation if the delay is less than the guard interval duration. At this time, the guard interval is removed by use of a “window” corresponding to the effective symbol duration, and the waveform within the effective symbol duration is used for the demodulation.
However, when a delayed wave having an excess delay longer than the guard interval arrives under the influence of multipaths, the error rate increases greatly. In such a case, the guard interval duration must be increased; however, the increased guard interval duration increases redundancy of communications, thereby lowering communication efficiency.